The low value, \(D_n4000\sim 1\) , of the first peak signifies
the existence of a very young stellar population in the central parts of the host galaxies.
Note that SDSS fiber size limits the aperture
over which the light from a galaxy is collected.
As shown in [1]} and [2]},
the stellar age corresponding to \(D_n4000\sim 1\)  is
typically smaller than \(10^8\)  years, indicating that
the stars in the central parts of these galaxies formed
through short bursts. Because of the short time scale, galaxies
observed with such a young stellar population are rare,
which may explain the absence of the peak in the PDFs shown
in Figure REF . The peak at \(D_n4000=1\) ,
therefore, suggests that galaxies with strong current star formation
have a strong tendency to be AGNs hosts. This is consistent with
the result of [2]}, who found that the fraction of AGN hosts
among star-burst galaxies is high, and suggests that the process
associated with a star burst may trigger AGN activities.
